Thus the known damping films have the effect of passively damping the vibrations transmitted to the airplane structure, albeit without succeeding in eliminating them. The damping achieved depends in particular on the vibration frequency of the rotor. The inventors observed that this damping decreased for elevated frequencies, especially above 20 Hz, and that it was also slight for frequencies close to 5 Hz.
A problem arises when the damping achieved is slight precisely for some frequencies that also correspond to resonance modes of the airplane fuselage. In this case, the slightly damped vibrations undergone by the fixed bearing brackets may induce, under certain flight conditions, fuselage vibrations sufficiently strong to affect the comfort and safety of the passengers. This problem has been observed, for example, in certain aircraft having a fuselage of great length and relatively small diameter, for engine vibration frequencies on the order of 5 Hz and in a certain part of the flight range corresponding to flying speeds faster than 350 km/h (or approximately 190 KCAS—“Knots Calibrated AirSpeed”—) and altitudes higher than 6,000 m.
Since the efficacy of damping films of SFD type is imperfect, supplementary active control measures have been employed to reduce the fuselage vibrations induced by motors having rotor(s). Thus, EP 1375343 describes a device that makes it possible to activate the aircraft control surfaces in response to vertical and horizontal accelerations undergone by at least two engines symmetric relative to the fuselage. The accelerations undergone by each of the two engines are measured by accelerometers positioned on the engine, and the control instruction defining the orientation of the control surfaces is calculated by means of the measured accelerations and of a table of relations pre-established on the basis of the aeroelastic model of the aircraft.
Such a device makes it possible to reduce the fuselage vibrations even more. Nevertheless, it is still insufficient to guarantee a high level of passenger safety and comfort in a large-size aircraft, where strong vibrations may be felt from 5 Hz on in cruising phase, especially in the case of turbulences.
In addition, other problems arise:                to date no means exists for predicting with certainty the frequency and amplitude of vibrations induced by the engines or other rotating devices on the fuselage of a new aircraft version; these vibrations are evaluated during test flights of the aircraft, in a stage when any modification of the aircraft becomes complex,        because of this fact, the designers are incapable of predicting, upstream, adequate correction solutions that lead to only few structural modifications of the aircraft if such vibrations occur.        